The invention relates to parallelogram transmission used for producing a pressing force, particularly in household refuse compactors.
More particularly, the invention relates to parallelogram transmissions such as of the kind wherein the transmission is itself driven by a displacing member which produces a substantially straight-line motion, such as a screw spindle or the like, with the latter in turn being driven by an electromotor in whose current path there is provided a switching device operative for either turning the drive motor off or reversing its direction when a predetermined motor current and/or a predetermined pressing force has been reached.
With known parallelogram transmissions of this general type, to the extent that they are employed for force transmission and not for pure motion transmission or motion generation, there is the disadvantage that the maximum force exerted by a ram connected to and driven through the parallelogram transmission is not constant for a given output torque of the drive motor which drives the transmission. In the first place, the transmission ratio of the parallelogram transmission is not a constant, but varies as a function of the transmission displacement, i.e., varies as the ram moves from its starting position to its end-of-stroke position. The functional dependence of the transmission ratio upon the displacement of the parallelogram transmission, and upon the ram position, is essentially a tangential function. Accordingly, if it is assumed that the drive motor output torque is the same at the end of every ram stroke, from one ram stroke to the next, and if the successive ram strokes are of different length, then the maximum pressing force exerted by the ram (at the end of its stroke) will be greater for long strokes than for short strokes. Indeed, for long strokes the maximum pressing force exerted by the ram (at the end of its stroke) will be a multiple of the maximum pressing force which it exerts (at the end of its stroke) during the performance of short strokes.
This is particularly undesirable when the drive motor is turned off or reversed, thereby ending the ram stroke, when the drive motor output torque reaches a preselected maximum value. The drive motor output torque can be measured directly and compared against the preselected maximum value, but usually use is simply made of a switching device connected in the drive motor current path and operative for sensing the motor torque indirectly, by sensing the motor current, and for turning off or reversing the motor when the motor current reaches a preselected maximum value assumed to correspond to the preselected maximum torque.
Specifically, a problem is constituted by the selection of the motor output torque, or equivalently the motor current, at which the motor should be automatically turned off or reversed, i.e., at which the ram stroke should be terminated. When the maximum value of the motor torque has been preselected, then the maximum pressing force which can be developed by the ram during the performance of its stroke increases as the ram moves farther and farther from its starting position. If a certain minimum pressing force is required over the whole range of movement of the ram, or over at least the middle part of the range of movement of the ram, then the dimensioning of the motor, transmission and stress-bearing components of the apparatus must be based upon the maximum compacting force which can be developed by the ram when it is still not very far from its starting position. But if this is done, and if the ram is made to perform a series of strokes of different respective lengths, then the maximum compacting force which can be exerted by the ram (at the end of its stroke) will for longer strokes be markedly greater than really necessary for the pressing operation, while for the shorter strokes the maximum compacting force which can be exerted by the ram (at the end of its stroke) will be only just sufficient for satisfactory pressing.
It is possible to use such a parallelogram transmission to move the compacting ram of a household refuse compactor, also using the abovedescribed expedient of automatically shutting off or reversing the drive motor and thereby ending the ram stroke, when the drive motor output torque or else the drive motor current reaches a preselected value. However, if this is done, then the resulting dependence of the maximum force which can be exerted by the compacting ram upon the position of the ram relative to its starting position, proves to be particularly disadvantageous, because of the special operating requirements involved.
This is because it is usual to compact the refuse in the container of the compactor in a layer-by-layer fashion; when the container is almost filled with a layer of loose refuse above layers of compacted refuse, the compacting ram is made to perform a stroke, so as to compact the loose refuse. The upper section of the container, as a result of such compacting, becomes free for the receipt of additional loose refuse. When additional loose refuse has substantially filled the upper section of the container, the compacting ram is made to perform another stroke, again compacting the loose refuse. This operation is repeated until the container of the compactor is filled to a predetermined level with compacted refuse, whereupon the container is entirely emptied or replaced by an empty container.
With this layer-by-layer compacting, the successive ram strokes are of shorter and shorter length, due to the growing height of the compacted refuse. Accordingly, the maximum compacting force developed by the ram (at the end of its stroke), during a series of successive progressively shorter strokes, reaches the highest value only during the first such stroke, i.e., only once. Nevertheless, it is necesssary that the parts of the refuse compactor which are subjected to mechanical stresses during the compacting operation be dimensioned to withstand the stresses arising during the exertion of the greatest compacting force, namely at the end of the first and longest stroke. This highest compacting force is a multiple of the force sufficient for satisfactory compacting of the refuse. Accordingly, the parts of the compactor subjected to mechanical stresses must be much more stable, strong and expensive than would be necessary if the maximum compacting force developed by the ram (at the end of its stroke) were the same from one ram stroke to the next and had only the value necessary for the desired degree of compacting effectiveness.
Likewise, if it is desired to limit the compacting force which can be developed by the ram to a value just sufficient for satisfactory compacting of the refuse, then one must recognize and accept that the degree of compaction will progressively decrease from one ram stroke to the next. This is because the aforedescribed shut-off or reversing device for the drive motor causes the drive motor output torque to be the same at the end of each successive ram stroke, despite the progressive decrease of the stroke length from one stroke to the next. Accordingly, the maximum compacting force exerted by the ram (at the end of the ram stroke) will become smaller and smaller, from one ram stroke to the next, the exact rerlationship between the length of the ram stroke and the maximum compacting force developed at the end of the respective stroke being the essentially tangential relationship between the displacement of the parallelogram transmission and its transmission ratio. But if this approach is taken -- i.e., limiting the compacting force which can be developed by the ram to a value just sufficient for satisfactory compacting of the refuse -- then, during the last ram strokes in a series of such strokes, a compacting force sufficient for satisfactory compacting and for the crushing of strong and hard components of the refuse, such as empty cans, bottles and the like, can no longer be achieved.
If, in contrast, the household refuse compactor and the parallelogram transmission thereof are dimensioned to ensure that the minimum necessary compacting force will always be reached during operation, despite the progressive decrease of the ram stroke length, then, besides the above-discussed disadvantageous effect upon manufacturing costs and upon the weight of the apparatus, there becomes necessary a very disadvantageous overdimensioning of the parallelogram transmission and of all the stress-bearing components of the refuse compactor, as well as substantially larger dimensions for the drive motor of the compactor. All this is disadvantageous in terms of energy consumption, space requirements and weight of the apparatus.
With another apparatus already known and available commercially, an attempt has been made to eliminate the difficulties associated with the transmission-ratio/displacement curve of the particular parallelogram transmission employed, by making use of helical tension springs. These tension springs are arranged parallel to the screw spindle in such a manner as to become relieved as the compacting ram descends and to become stressed again when the ram performs its return movement. These springs are intended to compensate for the undesirable transmission-ratio/displacement curve of the parallelogram transmission by supplementing the initially low compacting force. However, the increase of the compacting force afforded by the helical springs turns out to be very unsatisfactory, because the screw spindle drive serving to move the parallelogram transmission is self-locking, so that the stressed helical springs contribute to a substantial increase of the friction in the entire drive and transmission arrangement.
The use of helical springs to compensate for the lower compacting force during the shorter ram strokes has the further disadvantage that, when the compacting ram is in the raised or rest position, the parallelogram transmission is subjected to the full stressing of the helical springs. This is detrimental for the mountings and moving parts of the transmission. Furthermore, the use of helical springs of the requisite strength and length can detrimentally influence the dimensions requisite for the parallelogram transmission.
Because of the aforedescribed displacement/transmission-ratio curve of the parallelogram transmission, such parallelogram transmissions are only seldom employed for driving household refuse compactors, even though in this particular application the parallelogram transmission, due to the small space it takes up when the compacting ram is in the raised or rest position, is of great advantage and markedly superior to the otherwise conventional use of a screw spindle for direct drive of the compacting ram.